maybe.hh: add clear method

maybe.hh

@@ -42,9 +42,8 @@ struct maybe {
         new (&memory) T(*other.as_ptr());
         is_init = true;
       }
-    } else if (is_init) {
-      as_ptr()->~T();
-      is_init = false;
+    } else {
+      clear();
     }
     return *this;
   }
@@ -59,9 +58,8 @@ struct maybe {
         new (&memory) T(std::move(*other.as_ptr()));
         is_init = true;
       }
-    } else if (is_init) {
-      as_ptr()->~T();
-      is_init = false;
+    } else {
+      clear();
     }
     return *this;
   }
@@ -108,6 +106,13 @@ struct maybe {
     return is_init ? *as_ptr() : v;
   }
 
+  void clear() {
+    if (is_init) {
+      as_ptr()->~T();
+      is_init = false;
+    }
+  }
+
 private:
   T *as_ptr() { return reinterpret_cast<T *>(&memory); }
   const T *as_ptr() const { return reinterpret_cast<const T *>(&memory); }